The use of mobile communications networks has increased over the last decade. Operators of mobile communications networks have increased the number of base stations in order to meet an increased request for service by users of the mobile communications network. The base stations are typically coupled to an (active) antenna array. The radio signals are typically relayed into a cell of the mobile communications network, and vice versa. It is of interest for the operator of the mobile communications network to reduce the running costs of the base stations. It is one option to implement the radio system as an antenna embedded radio system. With the antenna embedded radio system formed as active antenna array some of the hardware components of the radio system may be implemented on a chip. The active antenna array therefore reduces the costs of the base station. Implementing the radio system as the antenna embedded radio system reduces space needed to house the hardware components of the base station. Power consumption during normal operation of the radio system is substantially reduced when implementing the antenna embedded radio system.
It is of interest to provide a reliable quality of service to an individual user of the mobile communications network given the increase in the number of users. Several techniques have been suggested in order to deal with the increased number of users within the mobile communications network. One of the several techniques comprises beam forming capabilities in order to direct a beam relayed by the active antenna array in different directions to improve service coverage within the cells of the mobile communications network. The beam forming techniques rely on defined phase and amplitude relations between several ones the antenna elements of the active antenna array. A transmit path and/or a receive path is associated with at least one antenna element. Calibration of the transmit paths and/or the receive paths is required to provide the defined phase, amplitude and delay relationship between the individual ones of the antenna elements. The calibration allows the estimation of a phase, amplitude and delay deviation accumulated along individual transmit paths of the active antenna array. Likewise the calibration comprises estimating phase, amplitude and delay deviations accumulated along individual ones of the receive paths. In a second step the phase, amplitude and delay deviation accumulated along the transmit paths can be corrected. An appropriate phase and amplitude change may be applied to the individual transmit/receive paths to yield the defined phase and amplitude relationship between the individual transmit/receive paths of the active antenna array, in order to allow for beam forming techniques.
In a modern mobile communications network a payload signal is provided as a packetized payload signal to the active antenna array. Packets of the packetized payload signal have a defined temporal order when the packetized payload signal is provided to the digital radio interface. Within the active antenna array some (data) processing may be applied to the packetized payload signal. The (data) processing typically comprises the packetized payload signal passing through several buffers and clock domains that are synchroized by PLLs. With the data processing the timing of the packet stream may change each time the system is restarted (reset). In the prior art, with non-packetized signals, it was possible and common practise to calibrate the transmit paths along which the non-packetized payload signal travels when being relayed by the radio station during manufacture of the radio station.
A delay experienced by a radio signal reaching the digital radio interface until a corresponding radio signal is relayed by antenna elements of the active antenna array is of interest for a coherent relaying of the active antenna array. The delay affects a phase relation between individual ones of the antenna elements as well as position based services. The delay is affected by any change in cable length and the like.
In the prior art it was necessary to recalibrate the active antenna array whenever a component of the active antenna array, for example, a cable, was replaced. The recalibration in the prior art is expensive and time consuming.
U.S. Pat. No. 6,693,588 B1 (assigned to Siemens) discloses an electronically phase-controlled group antenna. The electronically phase-controlled group antenna is calibrated using a reference point shared by all of reference signals. In the downlink, those reference signals which can be distinguished from one another are simultaneously transmitted by individual antenna elements of the group antenna and are suitably separated after reception at the shared reference point.
The Siemens system of U.S. '588 discloses a fixed spatial arrangement of the antenna elements.
FIG. 1a shows a passive antenna array 1a according to the prior art. A base station 5 provides a base station signal 7 to the passive antenna array 1a. A digital interface carries the base station signal 7 between the base station 5 and a central base band processing unit 10 of the passive antenna array 1a. The central base band processing unit 10 forwards a transmit signal Tx to a power amplifier 60 in order to amplify the transmit signal Tx. It is to be understood that the transmit signal Tx is typically provided in a transmit band of the mobile communication system. The signal leaving the central base band unit 10 is a transmit signal in the analogue domain. The transmit signal Tx entering the amplifier 60 requires an up-converting into a transmit band of the passive antenna array 1a. The transmit signal Tx will further require a digital-to-analogue conversion, if the transmit signal Tx is in the digital domain. The digital-to-analogue conversion is then carried out by a digital-to-analogue converter (not shown) prior to the amplification by the amplifier 60. The analogue transmit signal leaving the amplifier 60 is forwarded to individual transmit paths. Each of the transmit paths comprises a duplex filter 25-1, 25-2, . . . , 25-N forwarding the analogue transmit signals to an individual one of the antenna elements 85-1, 85-2, . . . , 85-N. It is to be noted that more than one individual antenna element 85-1, 85-2, . . . , 85-N may be coupled to an individual one of the duplex filters 25-1, 25-2, . . . , 25-N. Before entering the individual duplex filters 25-1, 25-2, . . . , 25-N the analogue transmit signal passes through a passive feeder network 40a. The passive feeder network 40a imposes a fixed phase, amplitude and/or delay relation between individual ones of the transmit paths terminated by the individual ones of the antenna elements 85-1, 85-2, . . . , 85-N. The passive feeder network 40a provides only little flexibility in terms of beam shaping. Any change of components within the passive feeder network 40a will require a recalibration of the paths from the amplifier 60 to the individual ones of the duplex filters 25-1, 25-2, . . . , 25-N. It is to be understood that individual ones of the transmit paths run from the amplifier 60 across the passive feeder network 40a and an individual one of the duplex filters 25-1, 25-2, . . . , 25-N and are terminated by an individual one of the antenna elements 85-1, 85-2, . . . , 85-N.
Individual receive paths of the passive antenna array 1a run from the individual antenna elements 85-1, 85-2, . . . , 85-N via the duplex filters 25-1, 25-2, . . . , 25-N and the passive feeder network 40a reaching a receive amplifier 70 as a general receive signal Rx. The general receive signal Rx is formed from individual receive signals received at the antenna elements 85-1, 85-2, . . . , 85-N combined by the passive feeder network 40a. The feeder network 40a imposes a fixed phase, amplitude and delay relation between the receive signals received at individual ones of the antenna elements 85-1, 85-2, . . . , 85-N. Therefore beam forming capabilities for the individual receive signals are limited by the passive feeder network 40a. 
The receive signal Rx is in the analogue domain. Individual receive signals from the antenna element may have undergone a filtering by the duplex filters 25-1, 25-2, . . . , 25-N as is known in the art. The receive signal Rx is amplified by the receive amplifier 70 and analogue-to-digital transformed using an analogue-to-digital converter (not shown), for example, a sigma-delta analogue-to-digital converter. The signal reaching the central base band processing unit 10 from the receive amplifier 70 is typically in the base band of the passive antenna array 1a. The receive signal from the receive amplifier 70 may be in an intermediate frequency band between a base band of the passive antenna array 1a and a transmit band of the passive antenna array 1a. The central base band processing unit 10 may impose some digital signal processing such as filtering to the digital receive signal and forwards the digital receive signal in the base band to the base station 5.
FIG. 1b shows a variant of the active antenna array 1a according to the prior art. A system as depicted in FIG. 1b is typically equivalent to combining a prior art remote radio head (RRH) with a known base station antenna within a common housing. The base station signal 7 comprises the receive signal from the central base band processing unit 10 being forwarded to the base station 5. In FIG. 1b the duplex filters 25-1, 25-2, . . . , 25-N of the individual transmit paths of FIG. 1a are replaced by a single duplexer 25. It will be appreciated that the system of FIG. 1b is more cost-efficient than the system depicted in FIG. 1a. 
The transmit signals and the received signal between the base station 5 and the central base band processing unit 10 are forwarded along a digital interface. The transmit signals and/or the receive signals may be provided in an in phase component I and a quadrature component Q. The in phase component I and the quadrature component Q may be provided according to a standard format set by the open base station architecture interface (OBASI) or in a common protocol radio interface (CPRI) format, but are not limited thereto.
FIG. 2 shows an active antenna array 1a according to the prior art. The active antenna array 1a of FIG. 2 does not comprise the passive feeder network 40a as shown in FIG. 1. Instead the antenna elements 85-1, 85-2, . . . , 85-N are terminating transceiver units 20-1, 20-2, . . . , 20-N. The transceiver units 20-1, 20-2, . . . , 20-N comprise amplifiers 60-1, 60-2, . . . , 60-N for each one of the transceiver units 20-1, 20-2, . . . , 20-N. Likewise the transceiver units 20-1, 20-2, . . . , 20-N comprise an individual receive amplifiers 70-1, 70-2, . . . , 70-N for each one of the transceiver units 20-1, 20-2, . . . , 20-N. The central base band processing unit 10 forwards individual transmit signals Tx-1, Tx-2, . . . , Tx-N from the central base band unit 10 to the individual amplifiers 60-1, 60-2, . . . , 60-N. The individual transmit signals Tx-1, Tx-2, . . . , Tx-N are typically in the analogue domain and in the transmit band of the active antenna array 1a. A digital to analogue conversion is typically carried out by the central base band processing unit 10, as explained above. The receive signal received at the individual antenna elements 85-1, 85-2, . . . , 85-N are amplified at the individual receive amplifiers 70-1, 70-2, . . . , 70-N and forwarded as individual receive signals Rx-1, Rx-2, . . . , Rx-N to the central base band processing unit 10. The individual receive signals Rx-1, Rx-2, . . . , Rx-N are combined by the central base band processing unit 10. The combining of the individual receive signals Rx-1, Rx-2, . . . , Rx-N is carried out in the base band domain. The individual receive signals Rx-1, Rx-2, . . . , Rx-N are in the analogue domain. The central base band processing unit 10 typically performs an analogue-to-digital conversion. The central base band processing unit 10 combines the individual receive signals Rx-1, Rx-2, . . . , Rx-N into a global receive signal, the global receive signal is typically forwarded to the base station 5.
The individual transmit signal Tx-1, Tx-2, . . . , Tx-N is in the analogue domain and the transmit band of the active antenna array 1a. The individual transmit signals Tx-1, Tx-2, . . . , Tx-N are generated by the central base band processing unit 10. The splitting into the individual transmit signal Tx-1, Tx-2, . . . , Tx-N may be carried out in a digital domain or in the analogue domain. The active antenna array 1a as depicted in FIG. 2 is known from phased array antennas used for example in RADAR applications or in magnetic resonance imaging.
The phased array antenna 1a can as well be formed in the receive case. Individual receive signals Rx-1, Rx-2, . . . , Rx-N are amplified by individual receive amplifiers 70-1, 70-2, . . . , 70-N and combined by the central base band processing unit 10 into a general receive signal. The combining into the general receive signal may be carried out in a digital domain and/or in the analogue domain. However, in order to operate such phased arrays, i.e. the active antenna array 1a as depicted in FIG. 2, phase, amplitude and delay relations between individual ones of the transceiver units 20-1, 20-2, . . . , 20-N need to be carefully calibrated in order to achieve an intended beam relayed by the active antenna array 1a. If the implementation of the active antenna array 1a is built up substantially in the analogue domain, the calibration of the active antenna array 1a is difficult and known solutions are often bulky and expensive.